Recently, in accordance with the increase in the amount of information, optical disks have been required to have a higher recording density. Optical disks with a higher recording density have been realized by increasing a linear recording density in an information recording layer of an optical disk or by providing tracks in a narrower pitch. To cope with realizing such optical disks with a higher recording density, it is necessary to decrease a beam diameter of a light beam focused onto the information recording layer of the optical disk.
It can be considered that a beam diameter of a light beam can be decreased by increasing a numerical aperture (NA) of an objective lens to which the light beam is directed, which serves as a focusing optical system in an optical pick-up device for recording/reproducing information on/from an optical disk, and by shortening a wave length of the light beam.
The shortening of the wave length of the light beam is considered as feasible by changing a light source from a red semiconductor laser to a blue-purple semiconductor laser which has been being put on the road to commercialization in a full scale.
On the other hand, as for an objective lens having a high NA, a technique in which a hemispherical lens is combined to an objective lens, and the objective lens is constituted by the two lenses (the two-group lens), so as to realize an objective lens having a high NA, has been proposed.
Meanwhile, generally in an optical disk, an information recording layer is covered with cover glass so as to be protected from dust and flaws. Therefore, a light beam passing through an objective lens in an optical pick-up device passes through the cover glass, and is focused onto the information recording layer located under the cover glass and forms a focus.
When the light beam passes through the cover glass, a spherical aberration SA is caused. The spherical aberration SA can be expressed as:SA∝d·NA4,  (1)
which means that the spherical aberration is proportional to a cover glass thickness d and the fourth power of the NA of the objective lens. Generally, the objective lens is designed so as to make up for the spherical aberration, so the spherical aberration of the light beam passing through the objective lens and the cover glass is sufficiently small.
However, if the thickness of the cover glass deviates from a predetermined value, the spherical aberration is caused in the light beam focused onto the information recording layer. Thus, the beam diameter is increased, causing a problem that information cannot be read/written correctly.
Besides, it is clear from the foregoing expression (1) that a spherical aberration error ΔSA increases with the increase of a thickness error Δd of the cover glass, resulting in that information cannot be read/written correctly.
Meanwhile, in order to have a higher information recording density in a direction of the thickness of the optical disk, a multi-layer optical disk formed by laminating information recording layers has been provided. For example, a DVD (Digital Versatile Disc) having two information recording layers has already been commercialized as a multi-layer optical disk. In an optical pick-up device for recording/reproducing information on/from such a multi-layer optical disk, it is necessary to focus a light beam so as to be sufficiently small, onto each of the information recording layers in the optical disk.
In an optical disk having multiple information recording layers as mentioned above, thicknesses from a surface of the optical disk (a surface of cover glass) to the respective information recording layers are different. Thus, spherical aberration caused when a light beam passes through the cover glass of the optical disk differs for each information recording layer. In this case, for example, the difference (error) ΔSA of the spherical aberrations caused in adjacently laminated information recording layers is proportional to a distance t between the adjacently laminated information recording layers (corresponding to the thickness d), which is obtained from the expression (1).
In the DVD having two information recording layers, the NA of the objective lens in the optical pick-up device is small, around 0.6. Consequently, it is clear from the foregoing expression (1) that, even if the thickness error Δd of the cover glass increases in some degree, the increase has little effect on the spherical aberration difference ΔSA.
Therefore, in a DVD device using a conventional optical pick-up device having a NA of around 0.6, the spherical aberration difference ΔSA caused by the thickness error Δd of the cover glass of the DVD is small, and thus the light beam can be focused onto each information recording layer so as to be sufficiently small.
However, even if the thickness error Δd of the cover glass is identical, a greater spherical aberration is caused with the increase of the NA. For example, when NA=0.85, an approximately four-fold spherical aberration is caused compared with the case where NA=0.6. Therefore, it is clear from the foregoing expression (1) that, the higher the NA becomes, as NA=0.85, the greater the spherical aberration is caused by the thickness error Δd of the cover glass.
Likewise, in the multi-layer optical disk, even if the distance t between the adjacently laminated information recording layers is identical, a greater spherical aberration difference ΔSA is caused with the increase of the NA of the objective lens in the optical pick-up device. For example, when NA=0.85, an approximately four-fold spherical aberration difference is caused compared with the case where NA=0.6. Therefore, it is clear from the foregoing expression (1) that, the higher the NA becomes, as NA=0.85, the greater the difference of the spherical aberrations of the respective information recording layers becomes.
Therefore, in an objective lens having a high NA, the effect by the spherical aberration error is not negligible, and it results in the deterioration of information reading accuracy. Hence, in order to realize a higher recording density using the objective lens having a high NA, it is necessary to correct the spherical aberration.
As techniques for correcting a spherical aberration, techniques disclosed in Japanese Unexamined Patent Publications No. 2000-155979 (Tokukai 2000-155979, published on Jun. 6, 2000: reference 1), No. 2000-182254 (Tokukai 2000-182254, published on Jun. 30, 2000: reference 2), No. 2000-171346 (Tokukai 2000-171346, published on Jun. 23, 2000, U.S. patent application Ser. No. 09/456,414, applied on Dec. 8, 1999: reference 3), etc. can be considered.
The reference 1 discloses a technique that, in a light beam on a return path, which is reflected from an optical disk and to be focused, only a part of the light beam which passes through a region between two concentric circles having different radii centered on a light axis of the light beam (a region in a half-ring shape) is focused so as to detect a spherical aberration, and the spherical aberration is corrected in accordance with the detected result.
The reference 2 discloses a technique that, a light beam on a return path, which is reflected from an optical disk and to be focused, is separated by a hologram device into a light beam close to a light axis of the foregoing light beam and a light beam outside the light beam close to the light axis of the foregoing light beam, and the two light beams are focused so as to detect a spherical aberration, and the spherical aberration is corrected in accordance with the detected result.
The reference 3 discloses a technique for detecting a spherical aberration when a light beam is focused onto an information recording layer of an optical disk, by utilizing the difference in a focus position of a part of the light beam close to a light axis and a focus position of a part of the light beam outside the part of the light beam close to the light axis, which results from the spherical aberration, and correcting the spherical aberration in accordance with the detected result.
However, the foregoing references 1 through 3 have the following problems.
In the reference 1, the light beam passing through the half-ring shaped region in the region between the two concentric circles having different radii centered on the light axis of the light beam, is utilized as a light beam for detecting the spherical aberration. The half-ring shaped region is a region including an extreme value of a curve representing a wave front of the light beam, and the light beam passing through the region is focused onto a focus position of a focused light beam in an ideal wave front having no spherical aberration. Therefore, the technique disclosed in the reference 1 cannot be adopted to a method for detecting a spherical aberration error signal by utilizing a focus position of a light beam.
In the references 2 and 3, the spherical aberration is detected by detecting a deviation in the focus positions of the separated light beams. Thus, unless the light beam is appropriately separated, the difference between the positions where the separated light beams have the minimum spot diameters, respectively, is reduced, and thus the amount of the deviation in the focus positions of the separated light beams is reduced, failing to detect the spherical aberration sensitively.